Add support for theme family-specific syntax mapping overrides (#3551)

This PR adds support for adding a specific set of mappings from Zed
syntax tokens to VS Code scopes for a particular theme family.

We can use this as a fallback when we aren't otherwise able to rely on
the mappings in the theme importer, as sometimes it isn't possible to
make a specific enough matcher that works across all of the themes.

Release Notes:

- N/A

crates/theme_importer/Cargo.toml

@@ -10,7 +10,7 @@ publish = false
 anyhow.workspace = true
 convert_case = "0.6.0"
 gpui = { package = "gpui2", path = "../gpui2" }
-indexmap = "1.6.2"
+indexmap = { version = "1.6.2", features = ["serde"] }
 json_comments = "0.2.2"
 log.workspace = true
 palette = { version = "0.7.3", default-features = false, features = ["std"] }